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[PATCH v3] xen/pass-through: merge emulated bits correctly

From: Chuck Zmudzinski
Subject: [PATCH v3] xen/pass-through: merge emulated bits correctly
Date: Wed, 29 Jun 2022 02:05:23 -0400

In xen_pt_config_reg_init(), there is an error in the merging of the
emulated data with the host value. With the current Qemu, instead of
merging the emulated bits with the host bits as defined by emu_mask,
the emulated bits are merged with the host bits as defined by the
inverse of emu_mask. In some cases, depending on the data in the
registers on the host, the way the registers are setup, and the
initial values of the emulated bits, the end result will be that
the register is initialized with the wrong value.

To correct this error, use the XEN_PT_MERGE_VALUE macro to help ensure
the merge is done correctly.

This correction is needed to resolve Qemu project issue #1061, which
describes the failure of Xen HVM Linux guests to boot in certain
configurations with passed through PCI devices, that is, when this error
disables instead of enables the PCI_STATUS_CAP_LIST bit of the
PCI_STATUS register of a passed through PCI device, which in turn
disables the MSI-X capability of the device in Linux guests with the end
result being that the Linux guest never completes the boot process.

Fixes: 2e87512eccf3
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1061
Buglink: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=988333

Signed-off-by: Chuck Zmudzinski <brchuckz@aol.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
v2: Edit the commit message to more accurately describe the cause
of the error.

v3: * Add Reviewed-By: Anthony Perard <anthony.perard@citrix.com>
    * Add qemu-stable@nongnu.org to recipients to indicate the patch
      may be suitable for backport to Qemu stable

Thank you, Anthony, for taking the time to review this patch.

 hw/xen/xen_pt_config_init.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/xen/xen_pt_config_init.c b/hw/xen/xen_pt_config_init.c
index cad4aeba84..21839a3c98 100644
--- a/hw/xen/xen_pt_config_init.c
+++ b/hw/xen/xen_pt_config_init.c
@@ -1966,10 +1966,10 @@ static void 
xen_pt_config_reg_init(XenPCIPassthroughState *s,
         if ((data & host_mask) != (val & host_mask)) {
             uint32_t new_val;
-            /* Mask out host (including past size). */
-            new_val = val & host_mask;
-            /* Merge emulated ones (excluding the non-emulated ones). */
-            new_val |= data & host_mask;
+            /* Merge the emulated bits (data) with the host bits (val)
+             * and mask out the bits past size to enable restoration
+             * of the proper value for logging below. */
+            new_val = XEN_PT_MERGE_VALUE(val, data, host_mask) & size_mask;
             /* Leave intact host and emulated values past the size - even 
              * we do not care as we write per reg->size granularity, but for 
              * logging below lets have the proper value. */

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